1. Field of the Invention
The present invention relates to a mass flow sensor for detecting the flow rate or mass flow of a fluid (gas or liquid), and to a mass flowmeter comprising the sensor. More particularly, the present invention relates to a mass flow sensor which realizes accurate detection of the flow rate or mass flow of a medium even when the flow rate, flow direction, or temperature of the medium changes drastically, and which is suitable for detecting the amount of intake air flowing into an internal combustion engine; and to a mass flowmeter comprising the sensor.
2. Description of the Related Art
Mass flow sensors including at least one heater formed on a substrate have been known. Such mass flow sensors are used for detecting the flow rate of a fluid by means of, for example, the degree of cooling of the heater caused by the flow of the fluid, the power or voltage required for maintaining the heater at a constant temperature, or the change in the temperature of an element which receives heat from the heater.
Some of the aforementioned mass flow sensors have a function for determining the flow direction of a fluid, as well as a function for detecting the flow rate of the fluid.
For example, (1) Japanese Patent Application Laid-Open (kokai) No. 1-185416 discloses a thermal flowmeter for an internal combustion engine including two heaters and two temperature compensation resistors, in which each heater is controlled such that the difference in temperature between the heater and a fluid becomes constant, and the flow rate and flow direction of the fluid are detected by the difference between the voltages applied to the heaters.
(2) Japanese Patent Publication (kokoku) No. 3-52028 discloses a flowmeter in which two heaters are formed on an insulating thin film having a cavity below the film, in order to reduce power consumption and increase response speed.
(3) Japanese Patent Publication (kokoku) No. 5-7659 discloses a flow rate sensor including one heater and heat detection sensors provided on both sides of the heater, the heater and the sensors being formed on an insulating thin film having a cavity below the film, in which the heater is controlled by a temperature compensation resistor such that the difference in temperature between the heater and a fluid becomes constant, and the flow rate and flow direction of the fluid are detected on the basis of the difference in temperature between the sensors.
However, since the flowmeter according to (1) described above includes a semiconductor substrate provided below the heaters, the heat capacity of the flowmeter increases by the heat capacity of the substrate. Therefore, a large amount of power is consumed for maintaining the heaters at predetermined temperatures, and response and characteristics at the time of start-up are unsatisfactory. In the case of the flowmeter according to (2) described above, the temperature of a fluid is not taken into account. In the case of the flow rate sensor according to (3) described above, saturation of output occurs at a relatively low flow rate of a fluid, since the heat detection sensor provided upstream of the heater, the sensor predominantly determining the output, is drastically cooled to a temperature near the temperature of the fluid when the flow rate of the fluid increases.
The present invention contemplates solving the aforementioned problems. It is therefore an object of the present invention to provide a mass flow sensor which consumes a small amount of power, and exhibits excellent response and characteristics at the time of start-up; as well as a mass flowmeter comprising the sensor.
A first aspect of the invention provides a mass flow sensor comprising a semiconductor substrate including a space section formed of a cavity, a notch, and/or a concave portion; an insulating thin film supported by the semiconductor substrate and adapted to provide thermal and electrical insulation; two heaters formed on a portion of the insulation thin film below which the space section is provided; two temperature measurement resistors formed on a portion of the insulating thin film which is thermally insulated from the heaters; and a protective layer formed on the insulating thin film, the heaters, and the temperature measurement resistors.
Preferably, the temperature measurement resistors are formed outside a portion of the insulating thin film below which a space section is provided. Preferably, on the insulating thin film, one end terminal of each of the heaters is connected to one end terminal of the corresponding temperature measurement resistor. Preferably, the temperature measurement resistors are provided substantially symmetrically with respect to the position of the heaters and/or a line parallel to the direction of the flow passage of a fluid under measurement.
Preferably, the temperature measurement resistors are provided along a line different from a line passing through the heater, these lines being parallel to the direction of the flow passage. Preferably, the temperature measurement resistors are disposed so as to form an interlocked configuration.
The aforementioned xe2x80x9cspace sectionxe2x80x9d refers to a section including at least one of a cavity formed below a bridge structure, a notch formed by a cantilever structure, and a concave portion formed by a depression. The space section may be provided in arbitrary number.
The aforementioned xe2x80x9cinsulating thin filmxe2x80x9d may be formed from any material, so long as the film can provide thermal and electrical insulation between a semiconductor substrate and the heaters and temperature measurement resistors, which are formed on the film. Examples of the material include silicon compounds such as SiO2, Si3N4, and SiOxNy. The insulating thin film may be formed as a lamination film.
When a thin film or a lamination film is formed, combination of the material and the thickness of the film may be appropriately determined in consideration of chemical durability, thermal stability, process suitability, adhesion between the film and wiring layers such as heaters and temperature measurement resistors and between the film and the semiconductor substrate, and balance between strength and stress of the film when formed into a thin film member. The insulating thin film may be formed by means of an arbitrary method such as thermal oxidation, CVD, sputtering, or application.
The wiring material for forming the aforementioned xe2x80x9cheaterxe2x80x9d and xe2x80x9ctemperature measurement resistorxe2x80x9d preferably has a high temperature coefficient of resistance, and undergoes minimal change in resistance and temperature coefficient of resistance even when used repeatedly for a long period of time. Examples of the material satisfying the above conditions include Pt and Nixe2x80x94Cr. The method for forming the heater and temperature measurement resistor may be determined arbitrarily, and examples thereof include wet etching, dry etching, and lift off.
The aforementioned xe2x80x9csupportxe2x80x9d may be carried out arbitrarily, so long as the insulating thin film can be supported such that the form of the film is maintained. For example, the insulating thin film may be provided on two semiconductor substrates provided so as to form a space therebetween, such that the film is bridged between the substrates. Alternatively, one end of the insulating thin film may be supported; i.e., a cantilever structure may be employed. Furthermore, the insulating thin film may be provided on a semiconductor substrate having a through-hole of arbitrary shape such that the film covers the through-hole.
In each of the aforementioned aspects of the invention, the method for producing a mass flow sensor is not particularly limited. Examples of the method include a known micromachining technique.
A further aspect of the invention provides a mass flowmeter comprising a mass flow sensor according to the above and further comprising a circuit for maintaining, for each heater, a constant difference between the temperature of the heater and the temperature of a fluid under measurement which is detected by the temperature measurement resistor corresponding to the heater.
Preferably, the mass flowmeter comprises two bridge circuits, each bridge circuit including one of the heaters and the temperature measurement resistor corresponding to the heater; and two heater power source circuits for maintaining, for each heater, a constant difference between the temperature of the heater and the temperature of the fluid, on the basis of outputs of the bridge circuits, the outputs being voltages applied to the bridge circuits or currents flowing through the bridge circuits.
Optionally, the mass flowmeter comprises a circuit for calculating the mass flow of the fluid on the basis of a subtraction value obtained by subtracting one of the outputs from the other, and for determining the flow direction of the fluid on the basis of whether the subtraction value is positive or negative. Optionally, the mass flowmeter comprises a circuit for calculating the mass flow of the fluid on the basis of the larger of the outputs, and for determining the flow direction of the fluid on the basis of whether a subtraction value obtained by subtracting one of the outputs from the other is positive or negative. Optionally, the flowmeter comprises a circuit for calculating the mass flow of the fluid on the basis of a subtraction value obtained by subtracting one of the voltages applied to the heaters from the other or by subtracting one of the currents flowing through the heaters from the other, or on the basis of the larger of the voltages or the currents, and for determining the flow direction of the fluid on the basis of whether the subtraction value is positive or negative.